The improved firing mechanism may be incorporated in any of a wide variety of firearms. For purposes of this patent application the invention is described and illustrated in combination with a rifle having a receiver or frame, a gas-operated longitudinally slidable bolt carrier mounted in the frame, a bolt disposed in the carrier, a spring biased hammer engageable with a firing pin in the bolt, and a trigger assembly mounted in the receiver. The forward end of the bolt carrier terminates in a piston residing in a fixed gas cylinder which receives pressurized gas through a port from the barrel of the rifle. When the trigger is forced to the rear by the operator's finger the hammer is released so as to pivot forward and strike the rear end of the firing pin which is thus forced forward into engagement with the primer of a cartridge disposed in a chamber at the rear end of the barrel. Burning of the powder in the cartridge forces the bullet down the barrel past the gas cylinder port, and hot gases pass through the port into the cylinder so as to force the piston, bolt carrier and bolt to begin moving rearwardly. During the initial movement the bolt rotates slightly relative to the frame and carrier so as to become disengaged from locking lugs on the receiver. During continued rearward movement, the carrier strikes and passes over the hammer forcing it to pivot backwards until it is engaged and held in a cocked position by the trigger assembly. The front end of the bolt in succession pulls the empty shell from the chamber and ejects it from the rifle. The carrier then strikes the rear of the receiver and stops. A longitudinally disposed spiral spring having its rear end fixed with respect to the receiver and its front end engaged with the carrier is compressed by the rearward movement of the carrier. As soon as the carrier is stopped by striking the receiver the spring begins to force the carrier forward. As the carrier moves forward the bolt picks up a fresh cartridge from a magazine or clip and moves the fresh cartridge into the chamber. The bolt engages cam surfaces on the carrier causing the bolt to rotate in the carrier and become locked in the receiver against rearward movement. The carrier moves slightly forward thereby rendering the trigger mechanism functional, in the sense that the mechanism is now permitted to operate in its intended manner.
All of the above is broadly conventional in automatic and semi-automatic firearms and need not be described or illustrated in detail in the present patent application. Accordingly, the drawings are limited to the details of the special firing mechanism which is the subject of this invention, with the conventional features being either omitted or illustrated schematically. More in particular, the described and illustrated rifle is a modified rifle of a known basic design, specifically a Soviet military weapon, known as an AK-47, which is selectively operable in either a semi-automatic or a full automatic (constant rate of fire) mode. The unmodified rifle is fully described in a publication entitled "The AK-47 Assault Rifle" edited by Wyant La Mont (Normount Technical Publications, Wickenburg, Ariz., Copyright 1969 by Donald B. Mclean). A further existing firearm which is readily modified to incorporate the present invention is the M62/s, a Finnish military rifle.
Firing mechanisms for changing the rate of fire in automatic firearms, by creating a delay in the operation of the mechanism, are disclosed in U.S. Pat. Nos. 3,015,993, 3,029,708 and 3,236,154.